SRF301 - Effect of Vestibular Evoked Myogenic Potential Stimuli on Auditory Temporal Resolution

Abstract:
Vestibular-evoked myogenic potentials (VEMP), despite being elicited using intense signals, were deemed safe due to no significant effects on pure tone thresholds and otoacoustic emissions. Nevertheless, a cochlear synaptopathy following VEMP testing could still be a possibility. To unveil this, 30 healthy individuals underwent gap detection threshold (GDT) measurement several times before and after the 500-Hz tone burst evoked cervical and ocular VEMP. Less than one-fourth of VEMP ears had a significantly worse GDT at 5 minutes, which recovered by the end of the experimental paradigm. Therefore, VEMP appears a safe diagnostic tool with no signs of permanent cochlear synaptopathy.

Summary:
Vestibular-evoked myogenic potentials (VEMP) are elicited using high-intensity signals, often 125 dB peSPL or higher. Evidence from human experiments showing temporary cochlear hypofunction after the VEMP test using 133 and 130 dB peSPL (Krause et al., 2013; Stromberg et al., 2016), was concerning. However, recent reports found 125 dB peSPL safe as it caused no significant change in pure tone thresholds and otoacoustic emissions (Rodriguez, et al., 2018; Singh et al., 2019). Nonetheless, loud sounds can affect the auditory nerve terminals at the synapse, commonly called cochlear synaptopathy, despite no significant change in pure tone thresholds and otoacoustic emissions. However, the possibility of a post-VEMP cochlear synaptopathy remains unexplored. The measures of temporal processing, such as the Gap Detection Threshold (GDT), could prove valuable for detecting cochlear synaptopathy as they are affected by the pathologies of the synapse and the auditory nerve (Bhardwaj et al., 2015; Kumar et al., 2012). Therefore, the present study aimed at investigating the effects of VEMP-eliciting stimuli on GDT. In an experimental time-series research design, 30 young adults (age range: 18-35 years) underwent a 500-Hz tone burst evoked cervical and ocular VEMP using an evoked potential system and gap detection test using the Maximum Likelihood Procedure in MATLAB. GDTs were measured just before the VEMP test and at 4-time points (5 minutes, 1 hour, 24 hours, and 2 weeks) after it. In the case of a persistently higher GDT than the baseline, the participants were re-evaluated after 2 months. The change in GDT at the post-VEMP measurement points (tGDT) was compared using non-parametric statistics due to non-normal data distribution (Shapiro-Wilk’s test, p< 0.05). There was a significant difference in tGDT between the measurement points of 5 minutes and 2 weeks and between 1 day and 2 weeks (p < 0.012, alpha-corrected, Wilcoxon signed rank test after the Friedman’s test). This finding could be attributed to the “bounce and fatigue phenomena”. It is observed that the “bounce phenomena” usually occurs after 2 minutes of the stimulation offset, whereas the time course for the “fatigue or recovery phenomena” varies (Hirsh, 1958; Hughes, 1954; Noffsinger & Tillman, 1970). In the present study, nearly 23% of VEMP ears had significantly worse GDT at 5 minutes after exposure. However, everyone recovered to the pre-exposure GDT level by the end of the experimental paradigm, which syncs with a well-known finding that around one-fourth of the ears are susceptible to temporary damage even after a low level of exposure below the defined regulatory standards (Price, 1981). Thus, the results of the present study show that the acoustic stimuli presented at an intensity of 125 dB peSPL to elicit VEMP have the potential to influence the auditory system temporarily. Nevertheless, the comparison of the potential benefits of the VEMP test to its temporary ill-effect weighs the odds in favor of the VEMP test, especially given that cVEMP is the only possible assessment tool for the saccular function. Hence, we argue that, despite temporary changes in GDT, VEMP is a safe tool for the otolith function assessment.